The RGB−spectral conversion method has been used to quantitatively visualize proton diffusion phenomena that occur in a time on the order of 10 ms when an acidic ion−exchange resin is placed on an agar gel containing an acid–base indicator. The absorption spectra of the mixed indicator of bromothymol blue and methyl red were reproduced from two sets of color coordinates: RGB and L*a*b* values. We recorded a video of the color change of bromothymol blue−methyl red (BTB−MR)−containing agar gel in contact with the ion−exchange resin at a time interval of 10 ms. We successfully obtained the absorption spectra in a short time and predicted the pH values around the resin. Our results suggest that the spectral conversion method based on color coordinates has the potential for measuring the behavior of materials in nonequilibrium systems. Moreover, the present method would be one of novel sensing technologies for continuous monitoring of spectral change, which can be utilized as precise inline analysis.